In recent years, there has been an increasing need for a downsized structure of a ventilating fan and an air-conditioning device so as to be suitable for a limited installation space in a living or non-living environment. At the same time, there has been a growing demand for the devices to have improved air-blow efficiency and noise-reduced structure.
Hereinafter will be described conventional structures of a centrifugal impeller and a centrifugal air blower with reference to FIG. 14. Centrifugal air blower 101 of FIG. 14 has casing 107; centrifugal impeller 112 with a multi-blade structure; and electric motor 113. Casing 107 has air-intake plate 103; back plate 104 that faces air-intake plate 103; and side wall 106. Casing 107 is formed so that air-intake plate 103 and back plate 104 hold side wall 106 therebetween. Air-intake plate 103 has bell-mouthed air inlet 102. Formed into a spiral, side wall 106 has air outlet 105. Accommodated in casing 107, centrifugal impeller 112 has ring-shaped side plate 108; main plate 110; and a plurality of blades 111. Main plate 110 has draw part 109 formed into a cone that protrudes toward side plate 108. Centrifugal impeller 112 has a structure where blades 111 are held between side plate 108 and main plate 110. Besides, centrifugal impeller 112 is connected to rotary shaft 114 of electric motor 113. Electric motor 113 is fixed to back plate 104. Such structured centrifugal air blower 101 is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 3629690 (hereinafter, referred to Patent document 1).
In centrifugal air blower 101 with the aforementioned structure, a driving force fed from motor 113 to rotary shaft 114 rotates centrifugal impeller 112. The rotation of centrifugal impeller 112 allows intake air 115 to pass through air inlet 102 and flow into blades 111, increasing pressure. Intake air 115 flows out of blades 111 and passes casing 107. While passing casing 107, intake air 115 gradually changes the increased pressure from dynamic form to static form, and is discharged from air outlet 105 to the outside. In the process above, flow 116, which comes from blades 111 and passes casing 107, maintains high pressure. Therefore, flow 116 from blades 111 partly flows into the space between main plate 110 and back plate 104, generating backflow 117. If backflow 117 stagnantly stays around there, air-blow efficiency of centrifugal air blower 101 can be lowered. To prevent this, ventilation holes 118 are disposed in main plate 110. Having ventilation holes 118 allows backflow 117 to go back into blades 111, producing circulation flow 119. This prevents against the degradation of the air-blow efficiency. In addition, ventilation holes 118 suppress the intake air 115 from hitting draw part 109 and flowing into blades 111. Further, by virtue of ventilation holes 118, intake air 115 and circulation flow 119 are led to electric motor 113, by which electric motor 113 is cooled.
Here will be described another structure of conventional centrifugal air blower 101 with reference to FIG. 15. Centrifugal air blower 101 of FIG. 15 has a structure where a part of back plate 104 of casing 107 located outside centrifugal impeller 112 is expanded in the direction of the rotary shaft. Besides, the degree of the expansion with respect to the rotating direction gradually increases toward air outlet 105. Such structured centrifugal air blower 101 is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 2690005 (hereinafter, referred to Patent document 2).
The aforementioned structure of centrifugal air blower 101 allows the flow fed from centrifugal impeller 112 to have a wide channel. Besides, the channel gradually increases toward air outlet 105, allowing centrifugal air blower 101 to have an improved air-blow efficiency and noise-reduced structure.
However, the conventional structures of centrifugal impeller 112 and centrifugal air blower 101 disclosed in Patent document 1 have some problems. That is, flow 116 fed from blades 111 partly goes as backflow 117 between main plate 110 and back plate 104. As described above, the conventional structure has a ventilation hole so as to form circulation flow 119 to prevent a stagnant state of the backflow. At that time, however, flow 116 flown out from the periphery of main plate 110 collides with backflow 117. The collision of flows hinders blades 111 disposed adjacent to main plate 110 in exerting their functions appropriately, which invites degradation of blower efficiency of centrifugal air blower 101. Besides, the collision of flow 116 and backflow 117 allows circulation flow 119 to be flown back, in a turbulent state, into blades 111. This also contributes to degradation of blower efficiency of centrifugal air blower 101. Further, the collision of flow 116 from the periphery of main plate 110 and backflow 117 generates turbulence noise.
Responding to demands for size reduction of centrifugal impeller 112, manufacturers have tried a structure where a part of electric motor 113 is disposed at cone-shaped draw part 109; the arrangement depends on the size of motor 113 and centrifugal impeller 112, and at the same time, draw part 109 has a limitation in its height. The constraints above have been an obstacle to size reduction of centrifugal air blower 101.
On the other hand, according to conventional centrifugal air blower 101 introduced in Patent document 2, back plate 104 has an intricate structure, specifically, casing 107 is partly formed into a spiral. This allows centrifugal air blower 101 to have a complex structure; accordingly, to have low productivity and high production cost.    Patent document 1; Japanese Unexamined Patent Application Publication No. 3629690    Patent document 2; Japanese Unexamined Patent Application Publication No. 2690005